Electric energy is known to be produced using direct-drive wind turbines (i.e., the type comprising a blade assembly on which the wind blows, and a rotary electric machine comprising a rotor connected directly to the blade assembly). In this type of wind turbine, the rotary electric machine rotor rotates at the same (i.e., relatively slow) speed as the blade assembly, and the wind turbine has the advantages of not requiring a transmission between the blade assembly and the rotor, and of being relatively cheaper to produce and maintain.
This design, however, calls for large-diameter electric machines, especially in the case of permanent-magnet rotary electric machines, which call for a large plurality of poles to compensate for the relative slow rotation speed of the blade assembly.
Moreover, the need for high-power wind turbines significantly affects the size of the rotary electric machines.
The trend is therefore towards increasing the diameter of wind turbine rotary electric machines. Electric machines used on wind turbines, however, must have specific characteristics: they must be relatively lightweight and relatively easy to access for maintenance, and their component parts must be relatively easy to assemble, disassemble and transport at both the first-time installation and repair stages.
One type of rotor described in PCT Patent Application No. WO 2002/099950 comprises a hub designed to rotate about an axis of rotation; a tubular structure extending about the hub and designed to support a plurality of active segments arranged about the axis of rotation; and a spider-type radial structure for connecting the hub to the tubular structure. Though apparently extremely lightweight, this type of rotor is not relatively easy to assemble, and is extremely complicated to maintain.
U.S. Published Patent Application No. 2011/266909 discloses a wind turbine rotor tubular structure made of several sectors each for supporting active segments, whereas EP Patent No. 1,988,282 discloses a wind turbine stator tubular structure made of several sectors each for supporting active members.
The tubular structure is thus relatively easier to assemble, at both the first-time installation and maintenance stages.
This characteristic enables the sectors to be extracted and inserted axially.